The present invention relates to a wet press for a paper making machine and particularly to a press with several press nips and wherein the web is supported on its path between the nips, sometimes without a supporting felt belt.
A press of this type is known from U.S. Pat. No. 4,556,451. This press is comprised of four press rolls which form a continuous chain so that the web of paper is always wrapped around a press roll over a part of the circumference of the roll during passage through the press. A total of three press nips are formed by the four press rolls. Upstream of the four roll press, there is a two roll press which receives the paper web coming from the wire section of the paper making machine.
A large number of paper making machine press sections of quite different configurations are known. U.S. Pat. No. 2,694,348 discloses a press section in which the web of paper is first removed by a first felt belt or first felt from the wire section of the paper machine and is then conducted by that first felt through a first two roll press. A second felt belt, which also passes through the first two roll press, receives the web of paper and conducts it through a second two roll press. Thereupon the web of paper is taken up by a third felt belt which passes through the second two roll press. The third felt belt conducts the web of paper through a third two roll press and then further through a fourth two roll press. This press section thus has a total of four press nips. Felt belts are passed through all of the press nips simultaneously along with the web of paper. The web of paper is transported by the felt belts between the individual press nips, over a path on which a felt belt bridges over each free path together with the web of paper present on the felt belt or hanging from it.
In general, wet presses or entire wet press sections should be adapted to the high demands of high speed paper machines. Wet presses should have a high pressing efficiency, should take up as little space as possible, particularly in the direction of travel of the paper web and in the horizontal direction, and furthermore should be of inexpensive construction.
However, with the increasing speeds of modern paper making machines, technical problems become greater, and new problems must constantly be solved. Such problems occur, for instance, upon restarting the paper making machine after it has been stopped, when a narrow strip of paper or tail must be passed through the press section. Furthermore, the danger of the paper web tearing increases with increasing speed of the paper machine. Furthermore, with this increasing speed, the water removal capacity of the press section must be increased. That cannot be done without increasing the number of press nips. However, this requires more space in the direction of travel of the paper machine. Another problem is marking of the paper web by the press felt belts while the web is still moist. Finally, wearing of the press felt belts increases with increasing speed of the machine and increasing linear pressures in the press nips, so that the felt belts must be more frequently replaced by new ones, which naturally increases the cost.